There are many known power assist steering systems for automotive vehicles. Some provide steering assist by using hydraulic power and others by using electric power.
Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's applied torque to the vehicle steering wheel, and (ii) sensed vehicle speed.
In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied to the steering wheel by the vehicle operator. An electronic control system includes a torque sensor and uses the output of a vehicle speed sensor. A computer receives the output signals provided by both sensors. The computer controls the amount of the assistance provided by the motor in response to the applied steering torque and the sensed vehicle speed.
U.S. Pat. No. 4,415,054 (now U.S. Reissue Pat. No. 32,222, hereinafter, "the Drutchas steering gear") utilizes a D.C. electric assist motor driven through an H-bridge arrangement. The motor includes a rotatable armature encircling a steering member. The steering member has a thread convolution portion and a portion having straight cut rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball-nut drive arrangement in combination with the thread convolution portion of the steering member. A torque sensing device is coupled to the steering column to sense driver applied input torque to the steering wheel. The torque sensing device uses a magnet Hall-effect sensor arrangement for sensing relative rotation between the input and pinion shafts across a torsion bar. An electronic control unit monitors the signal from the torque sensing device and controls the electric assist motor in response thereto.
U.S. Pat. No. 4,660,671 discloses an electric controlled steering system that is based on the Drutchas steering gear. In accordance with the '671 patent, a D.C. motor is axially spaced from the ball-nut and is operatively connected thereto through a connection tube. The electronic control unit includes a plurality of diagnostic features that monitor the operation of the steering system. If an error in the operation of the electric steering system is detected, the electric assist system is disabled and steering reverts to an unassisted mode.
Hydraulic power assist steering systems have an inherent yaw damping characteristic during a steering maneuver. Such damping characteristic in a power assist steering system is particularly important when the vehicle is traveling at a relatively high speed. It is, therefore, desirable to provide such a yaw damping characteristic in an electric assist steering system that would, at least, simulate that provided by a hydraulic power assist steering system and, preferably, improve thereupon.
A prior art electric assist steering system provided damping by switching a load resistor across the electric assist motor when the vehicle speed exceeded a predetermined value. When the vehicle speed was below the predetermined value or when applied steering torque exceeded a predetermined amount, the resistor was disconnected from across the motor. This method of damping was used to control the yaw rate of the vehicle when the road wheels returned after a turn.